The field of the invention is the removal of sulfur from coal. In particular, the method is concerned with coal decomposition during which gaseous products of pyritic sulfur are evolved.
Iron disulfide (FeS.sub.2) is one of the major sulfur compounds found in coal. It can appear in two crystalline forms, the pyrite (cubic) and the marcasite (rhombic). Since most of the iron disulfide is usually in the pyrite form, the names "pyrites" and "pyritic sulfur" are often used to designate all of the FeS.sub.2, and are so used in this application.
The chemical reactivity of the two forms of pyrites is similar. Thermal decomposition of pyritic sulfur converts the iron disulfide to iron sulfide and free sulfur. The resulting nascent sulfur then reacts with other products. See, Attar, Fuel, 57:201-212 (1978). The sulfur is evolved as hydrogen sulfide (H.sub.2 S) and not as elemental sulfur.
In the pyrolysis of coal a mixture of volatile products is produced. Hydrogen is the principal reactant with the sulfur formed by the decomposition of the pyrites, resulting in the hydrogen sulfide (H.sub.2 S) decomposition product. See, Mazumdar et al., Fuel, 41:121 (1962). Mazumdar found that the reaction of sulfur with hydrogen has an appreciable rate even at temperatures as low as 170.degree. C. Hydrogen also reacts directly with pyrites to form ferrous sulfate and hydrogen sulfide. Huang et al., pages 290-304, in "Coal Desulfurization: Chemical and Physical Methods," Wheelock, editor, American Chemical Society, Washington, D.C. (1977). In the same publication, see also Haldipur et al., pages 305-320.
The reaction of hydrogen with pyrites begins around 300.degree. C. (Fleming et al., pages 267-279, in "Coal Desulfurization: Chemical and Physical Methods," cited above.) In the thermal decomposition of coal, hydrogen begins to be evolved at temperatures in the range of 300.degree.-400.degree. C. Therefore, the reactions forming hydrogen sulfide from pyrites are dependent on pyrolysis temperatures. Other reactions of iron sulfide require temperatures in excess of 800.degree. C. (See Attar, 1978, cited above.)
In addition to Huang et al. and Haldipur, et al., cited above, literature references relating to the removal of pyritic sulfur from coal include: Speight, "The Chemistry and Technology of Coal," Marcel Dekker, Inc., New York (1983); and Powell, J. Ind. Engr. Chem., 12:1069-1977 (1920).
Huang et al. studied Iowa coal containing pyrites. The amount of sulfur volatilized in heating the coal from 250.degree. C. to 700.degree. C. in an atmosphere of nitrogen was determined. Reported data indicates a steady increase in sulfur evolution between 300 and 500.degree. C. with some decrease in the rate of 500.degree. C. Speight generalized that hydrogen sulfide is released between 250 and 500.degree. C. Haldipur found that nearly 30% of the total sulfur was released in heating coal from 320.degree. C. to 400.degree. C. Powell studied several reactions involving sulfur and coal. He concluded that the decomposition of pyrite begins at 300.degree. C. and is complete at 600.degree. C., the maximum being between 400 and 500.degree. C. Powell also found that one-fourth to one-third of the organic sulfur is decomposed to H.sub.2 S below 500.degree. C.
As far as is known, the cited studies have not resulted in any commercial application for the desulfurization of coal. Pyrolytic decomposition of coal to produce a product gas has been conventionally carried out for many years, but such pyrolytic decomposition has been employed primarily with low sulfur coals.